Dysfunction in the molecular pathways that regulate synapse form and function leads to a number of neurological disorders, including epilepsy, autism and mental retardation. Our goal is to explore the molecular machinery that mediates synapse development and morphogenesis. This fundamental knowledge will be important for our understanding of neurological disease and for the conception of future therapeutic tools. Using the Drosophila neuromuscular junction (NMJ) as a genetic model system, we have discovered that miR- 8, a member of the highly conserved miR-200 family of microRNAs (miRNAs), is essential for the normal growth and complexity of the synapse. Animals lacking miR-8 display NMJ defects at different stages of development. During larval stages, when NMJs dramatically expand under control of multiple stimuli and regulatory pathways, miR-8 is required in muscle cells to promote the growth of presynaptic terminals. Our analysis suggests that miR-8 is required for the normal architecture of the cytomatrix which defines subsynaptic reticulum (SSR) of the NMJ, a structure analogous to the postsynaptic density marked by PSD-95 in mammals. Multiple screens to define downstream effectors suggest that miR-8 regulates the expression of several target genes implicated in synaptogenesis and cytoskeletal biology. To better define the mechanisms downstream of miR-8, we have shown that postsynaptic repression of the actin-associated protein Enabled (Ena) plays an important role in controlling NMJ growth in late larval stages, consistent with the localization of Ena to the SSR. Ena is predicted to be a direct target of miR-8, and controls aspects of cytoskeletal structure and dynamics during cell movement and cell junction formation, but its role(s) at the synapse are not understood. Preliminary data also indicate that although miR-8 is expressed in the central nervous system (CNS), its activity is somehow suppressed in neurons relative to other tissues. Moreover, genetic epistasis reveals that miR-8 is required for NMJ expansion induced by activation of a key presynaptic pathway that limits synapse morphogenesis (the Fragile-X Mental Retardation gene, FMR1), suggesting that some type of trans- synaptic communication is involved upstream of postsynaptic miR-8. Together, these findings reveal a fascinating mechanism that regulates synapse development, and a wonderful opportunity to exploit a powerful and well-defined model system to understand the logic of miRNA control over synapse form and function. However, many additional studies will be required to define the developmental, cellular and molecular mechanisms required for miR-8 to exert its effects at the NMJ.